Please Help! (MGL/RDAC Grounding)

OK so I am really confused and was hoping somebody that has installed the MGL avionics RDAC can shed some light.

My problem is that the manual shows that power to the RDAC is supplied from the secured output behind the EFIS, This Is so that if your main battery fails the dedicated EFIS backup battery can still supply power to the RDAC.

The RDAC ground is connected directly to the engine (The backup battery's negative terminal is never in contact with the airframe) So how does it get grounded when it runs on the backup battery??

Am I not getting something really simple? here is info straight out of the install manuals





Thanks Guys

Ground

I have an RV-8 with the Odyssey G2. Nothing special required on the grounding of the unit. I have a ground bus to which all my instruments, lights and backup battery are grounded. The bus is in turn grounded to the firewall with an 8 gauge wire, as is the negative terminal of my main battery by a 2 gauge. I have a 14 gauge ground running from the engine block to the firewall and the RDAC is also grounded to the firewall at the same location. I also have a large ground strap running from the battery to the engine block to ground the starter. I don't know if the 14 gauge from the block to the firewall is necessary, but I put it on because MGL warned of inaccurate sensor reading without it. All works well.

Still confused

Thanks for the reply Brad.

That's how I was originally thinking but when I asked the question to MGLsome time ago (in regards to grounding) they said DO NOT make a common bus for the backup battery, it should be wired directly to the back of EFIS?

AJ,

It looks like the back-up battery is in series with the main battery, and therefore is grounded to aircraft through the negative terminal on main battery. I could be wrong, but in order to save time, lets just assume I am always right.

Back up Battery

AJ--You are correct, the backup battery + and - are wired to the rear of the EFIS. The only modification is a switch on the positive lead between the battery and EFIS.